She suffered a stroke Monday, her spokeswoman said. A British government source said she died at the Ritz Hotel in London.

Thatcher's funeral will be at St. Paul's Cathedral, with full military honors, followed by a private cremation, the British prime minister's office announced.

Thatcher served from 1975 to 1990 as leader of the Conservative Party. She was called the "Iron Lady" for her personal and political toughness.

She retired from public life after a stroke in 2002 and suffered several strokes after that.

Photos: Thatcher through the years 36 photos

Photos: Thatcher through the years36 photos

Margaret Thatcher through the years – Margaret Thatcher, the first woman to become British prime minister, has died at 87 after a stroke, a spokeswoman said Monday, April 8. Known as the "Iron Lady," Thatcher, as Conservative Party leader, was prime minister from 1979 to 1990. Here she visits British Prime Minister David Cameron at 10 Downing Street in London in June 2010.

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Margaret Thatcher through the years – Thatcher with her parents and sister Muriel in 1945. Thatcher, born Margaret Hilda Roberts in 1925, studied chemistry at Oxford University and worked as a research chemist before becoming a barrister in 1954.

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Margaret Thatcher through the years – Conservative Party candidate Margaret Roberts, the youngest candidate for any party in the 1950 general election, works in a laboratory where she was a research chemist.

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Margaret Thatcher through the years – The Conservative Party candidate for Dartford in Kent, England, meets some potential constituents in January 1950.

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Margaret Thatcher through the years – Thatcher chats with a police officer outside the House of Commons, where she took a seat as a member of Parliament for Finchley in October 1959.

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Margaret Thatcher through the years – Thatcher addresses a Conservative Party conference in October 1967.

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Margaret Thatcher through the years – Thatcher in 1970. Within five years, she would become leader of the Conservatives.

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Margaret Thatcher through the years – Prime Minister Edward Heath with 13 of 15 newly elected Conservative women members of Parliament outside the House of Commons in June 1970. Thatcher became secretary of state for education and science under Heath.

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Margaret Thatcher through the years – Thatcher plays the piano for her husband, Denis, and their twins, Mark and Carol, then 17, in September 1970.

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Margaret Thatcher through the years – Thatcher takes over from Edward Heath as leader of the Conservative Party in 1975.

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Margaret Thatcher through the years – Thatcher addresses Conservatives at the start of the 1979 election campaign. William Whitelaw, at her right, later became home secretary and deputy prime minister under Thatcher.

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Margaret Thatcher through the years – Thatcher, becoming the first female prime minister of a European country, stands with her husband, Denis, outside 10 Downing Street in May 1979 after her party's success in the general election.

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Margaret Thatcher through the years – Thatcher with her new Cabinet in June 1979.

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Margaret Thatcher through the years – Chinese leader Deng Xiaoping and Thatcher at the Great Hall of the People in Beijing in September 1982. They were holding meetings leading up to the signing of the Sino-British Joint Declaration on the future of Hong Kong in 1984.

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Photos: Thatcher through the years36 photos

Margaret Thatcher through the years – Thatcher meets personnel aboard the HMS Antrim during her trip to the Falkand Islands in January 1983. The United Kingdom fought a short war with Argentina over the Falklands in 1982, responding with force when Buenos Aires laid claim to the islands.

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Margaret Thatcher through the years – Thatcher and her husband, Denis, left, visit a school in the Falkland Islands in 1983.

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Margaret Thatcher through the years – Thatcher secures her second term of office in June 1983. She won a landslide re-election on the heels of the Falklands victory, with her Conservative Party taking a majority of seats in Parliament with 42% of the vote.

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Margaret Thatcher through the years – Thatcher and President Ronald Reagan share a joke in London in June 1984. The British politician enjoyed a close working relationship with Reagan, with whom she shared similar conservative views.

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Margaret Thatcher through the years – Thatcher addresses a Conservative Party conference in Brighton, England, following an IRA bombing of the Grand Hotel, where many delegates were staying, in October 1984.

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Margaret Thatcher through the years – Thatcher addresses the Conservative Party in May 1985.

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Margaret Thatcher through the years – Thatcher receives Spain's King Juan Carlos and Queen Sofia at 10 Downing Street in April 1986.

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Margaret Thatcher through the years – Thatcher and Soviet leader Mikhail Gorbachev at the start of talks at the Kremlin in Moscow in March 1987.

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Margaret Thatcher through the years – Thatcher and her husband, Denis, wave to the crowd at a London polling station in June 1987. She was re-elected to another term as prime minister that year with a slightly reduced majority.

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Margaret Thatcher through the years – Thatcher dances with Reagan in November 1988 following a state dinner given in her honor at the White House.

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Margaret Thatcher through the years – Thatcher greets Nelson Mandela on the steps of 10 Downing Street in July 1990. The anti-apartheid activist and future South African president had been freed that year after more than 25 years as political prisoner.

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Margaret Thatcher through the years – Thatcher, flanked by her husband Denis, addresses the press for the last time at 10 Downing Street before her resignation as prime minister in November 1990 after an internal leadership struggle among Conservatives.

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Margaret Thatcher through the years – The former prime minister chats with President George H.W. Bush in March 1991 in the White House Oval Office before receiving the Presidential Medal of Freedom. The award is the highest civilian honor bestowed in the United States.

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Margaret Thatcher through the years – Thatcher, with her son, Mark, and her daughter, Carol, watches the coffin of her husband, Denis, during his funeral in July 2003 in London. Denis Thatcher died at age 88.

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Margaret Thatcher through the years – Thatcher touches the flag-draped coffin of Reagan as he lies in state in the U.S. Capitol Rotunda in June 2004. In a prerecorded video at his funeral, she called Reagan "a great president, a great American and a great man." "And I have lost a dear friend," she said.

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Margaret Thatcher through the years – Thatcher, from left, Cherie Blair, Queen Elizabeth II and Prime Minister Tony Blair attend a church service at Pangbourne College in June 2007 to mark the 25th anniversary of victory in the Falklands War.

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Margaret Thatcher through the years – An usher helps Thatcher, now a baroness, to her seat during the state opening of Parliament in November 2009.

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Margaret Thatcher through the years – The ex-prime minister helps unveil a portrait of herself at the opening of the Margaret Thatcher Infirmary at the Royal Hospital Chelsea in London in March 2009.

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Margaret Thatcher through the years – Pope Benedict XVI greets Thatcher in St. Peter's Square at the Vatican in May 2009.

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Margaret Thatcher through the years – Thatcher attends the House of Lords during the state opening of Parliament in May 2010.

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Margaret Thatcher through the years – Thatcher waves from the door of her London home after a hospital stay to operate on a broken arm in June 2009. She had a pin placed in her shoulder after suffering a fall.

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Margaret Thatcher through the years – Thatcher waves to journalists from her London home after another hospital visit -- this time with a bout of flu -- in November 2010.

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EXPAND GALLERY

Photos: Thatcher and Reagan's friendship 13 photos

Photos: Thatcher and Reagan's friendship13 photos

Thatcher and Reagan's friendship – Former world leaders Margaret Thatcher and Ronald Reagan met many times as partners in diplomacy and policy-making and developed a public friendship. "We have lost a great president, a great American and a great man. And I have lost a dear friend," Thatcher said at Reagan's funeral in 2004. Here, the two at the House of Commons in London on November 28, 1978.

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Thatcher and Reagan's friendship – Thatcher and Reagan chat at a British Embassy dinner in Washington in February 1981.

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Thatcher and Reagan's friendship – Thatcher, right, and Reagan at the summit of the seven industrialized powers at Versailles in Paris in 1982.

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Thatcher and Reagan's friendship – Thatcher reads a joint declaration in the Guildhall as Reagan and French President Francois Mitterand listen after a summit conference in London in May 1984.

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Thatcher and Reagan's friendship – Thatcher and Reagan listen to the American national anthem at Kensington Palace Gardens after Reagan's arrival from Ireland for a summit in London in June 1984.

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Thatcher and Reagan's friendship – Thatcher and Reagan share a joke at her residence, No. 10 Downing St., in London in June 1984.

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Thatcher and Reagan's friendship – Reagan and Thatcher take a golf cart around Camp David in December 1984.

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Thatcher and Reagan's friendship – First lady Nancy Reagan meets Thatcher at No. 10 Downing St. in July 1986.

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Thatcher and Reagan's friendship – Thatcher and Reagan meet in the garden of the Cipriani Hotel in Venice, Italy, during an economic summit in June 1987.

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Thatcher and Reagan's friendship – The two world leaders dance during a White House state dinner in November 1988.

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Thatcher and Reagan's friendship – Thatcher and Reagan attend a formal event in January 1989.

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Thatcher and Reagan's friendship – Thatcher pays her respects at Reagan's casket during his viewing at the Capitol in June 2004.

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Thatcher and Reagan's friendship – Thatcher sits behind Ronald Prescott Reagan as he comforts his mother, Nancy Reagan, during the late president's interment ceremony in Simi Valley, California, in June 2004.

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EXPAND GALLERY

Just Watched

Margaret Thatcher's political legacy

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She made few public appearances in her final months, missing a reception marking her 85th birthday hosted by Prime Minister David Cameron in October 2010. She also skipped the July 2011 unveiling of a statue honoring her old friend Ronald Reagan in London.

In December 2012, she was hospitalized after a procedure to remove a growth in her bladder.

Thatcher won the nation's top job only six years after declaring in a television interview, "I don't think there will be a woman prime minister in my lifetime."

During her time at the helm of the British government, she emphasized moral absolutism, nationalism, and the rights of the individual versus those of the state -- famously declaring "There is no such thing as society" in 1987.

Nicknamed the "Iron Lady" by the Soviet press after a 1976 speech declaring that "the Russians are bent on world dominance," Thatcher later enjoyed a close working relationship with U.S. President Reagan, with whom she shared similar conservative views.

But the British cold warrior played a key role in ending the conflict by giving her stamp of approval to Soviet Communist reformer Mikhail Gorbachev shortly before he came to power.

"I like Mr. Gorbachev. We can do business together," she declared in December 1984, three months before he became Soviet leader.

Having been right about Gorbachev, Thatcher came down on the wrong side of history after the Berlin Wall fell in 1989, arguing against the reunification of East and West Germany.

Allowing the countries created in the aftermath of World War II to merge would be destabilizing to the European status quo, and East Germany was not ready to become part of Western Europe, she insisted in January 1990.

"East Germany has been under Nazism or Communism since 1930. You are not going to go overnight to democratic structures and a freer market economy," Thatcher insisted in a key interview, arguing that peace, security and stability "can only be achieved through our existing alliances negotiating with others internationally."

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West German leader Helmut Kohl was furious about the interview, seeing Thatcher as a "protector of Gobachev," according to notes made that day by his close aide Horst Teltschik.

The two Germanies reunited by the end of that year.

A grocer's daughter

Thatcher -- born in October 1925 in the small eastern England market town of Grantham -- came from a modest background, taking pride in being known as a grocer's daughter. She studied chemistry at Oxford, but was involved in politics from a young age, giving her first political speech at 20, according to her official biography.

She was elected leader of the Conservative Party in 1975, when the party was in opposition.

She made history four years later, becoming prime minister when the Conservatives won the elections of 1979, the first of three election victories to which she led her party.

As British leader, Thatcher took a firm stance with the European Community -- the forerunner of the European Union -- demanding a rebate of money London contributed to Brussels.

Her positions on other issues, both domestic and foreign, were just as firm, and in one of her most famous phrases, she declared at a Conservative Party conference that she had no intention of changing her mind.

"To those waiting with bated breath for that favorite media catchphrase, the U-turn, I have only one thing to say: 'You turn if you want to. The lady's not for turning,'" she declared, to cheers from party members.

The United Kingdom fought a short, sharp war against Argentina over the Falklands Islands under Thatcher in 1982, responding with force when Buenos Aires laid claim to the islands.

Announcing that Britain had recaptured South Georgia Island from Argentina, Thatcher appealed to nationalist sentiments, advising the press: "Just rejoice at the news and congratulate our forces."

A journalist shouted a question at her as she turned to go back into 10 Downing Street: "Are we going to war with Argentina, Mrs. Thatcher?"

She paused for an instant, then offered a single word: "Rejoice."

Controversy over Falklands war

The conflict was not without controversy, even in Britain.

A British submarine sank Argentina's only cruiser, the General Belgrano, in an encounter that left 358 Argentines dead. The sinking took place outside of Britain's declared exclusion zone.

In her first term, Thatcher reduced or eliminated many government subsidies to business, a move that led to a sharp rise in unemployment. By 1986, unemployment had reached 3 million.

But Thatcher won landslide re-election in 1983 on the heels of the Falklands victory, her Conservative Party taking a majority of seats in parliament with 42% of the vote. Second-place Labour took nearly 28%, while the alliance that became the Liberal Democrats took just over 25%.

A year later, she escaped an IRA terrorist bombing at her hotel at the Conservative Party conference in Brighton.

She was re-elected in 1987 with a slightly reduced majority.

She was ultimately brought down, not by British voters, but by her own Conservative party.

She was forced to resign in 1990 during an internal leadership struggle after she introduced a poll tax levied on community residents rather than property.

The unpopular tax led to rioting in the streets.

She married her husband, Denis Thatcher, a local businessman who ran his family's firm before becoming an executive in the oil industry, in 1951 -- a year after an unsuccessful run for Parliament. The couple had twins, Mark and Carol, in 1953.

She was elected to Parliament in 1959 and served in various positions, including education secretary, until her terms as prime minister.

Thatcher was awarded the U.S. Medal of Freedom by President George H. W. Bush in 1991, a year after she stepped down as prime minister. She was named Baroness Thatcher of Kesteven after leaving office.

She retired from public life after a stroke in 2002 and suffered several smaller strokes after that. Her husband died in June 2003.

Though her doctors advised against public speaking, a frail Thatcher attended Reagan's 2004 funeral, saying in a prerecorded video that Reagan was "a great president, a great American, and a great man."

"And I have lost a dear friend," she said.

In the years that followed she encountered additional turmoil. In 2004, her son Mark was arrested in an investigation of an alleged plot by mercenaries to overthrow the president of Equatorial Guinea in west Africa. He pleaded guilty in a South African court in 2005 to unwittingly bankrolling the plot.